This present invention deals with the semiconductor type of memory devices capable of writing and reading information optically and having a multi-layer structure with multiple information layers as well as writing and reading arrangements for writing to and reading from such devices.
Rapid progress has been made in data communication via the Internet by using mobile digital devices that combine the functionality of a PC. Such progress necessitates the development of non-volatile miniaturized semiconductor memory devices, known as solid state memory. However, the current capacity of the solid state memory devices does not meet the demand of the multi-gigabyte capacity needed for various computing applications. It also should be noted that these devices are expensive to make. One type of such devices, namely, DRAM, is progressing remarkably and is entering an era of gigabit storage capacity. DRAM increases its integration density by migrating from the initial structure that includes a plurality of transistors in one cell to a structure that consists of one transistor per cell. In the gigabit memory era, however, a capacitance of the storage capacitor is not sufficient for additional capability increases even by adapting trenched and stacked structures. Various technology adaptations utilize high dielectric constant materials as insulating layers in capacitors, and yet many problems are unresolved even till now. The conventional DRAMs with the trenched or stacked structures, as well as DRAMs that employ high dielectric constant materials for storage capacitors, require complicated fabrication processes and expensive manufacturing equipment. A cost of some ten billion dollars is estimated to be necessary to realize a manufacturing line which can mass manufacture semiconductor memory devices with an improved integration density. Moreover, since the conventional DRAMs are designed essentially on the basis of a planar (two-dimensional) layout, they can not be further miniaturized by the means of lithography. Thus, a technical barrier appears in the conventional solid sate semiconductor memory technologies, and a breakthrough in the technical barrier is necessitated.
A possible solution of these problems relates to an approach that combines optical signal processing technology and microelectronics. Originally this approach has been proposed for the utilization in the optical computers. A main advantage of the optical processing is that an individual element of the system can communicate simultaneously with an enormous number of other elements. This advantage originates from the fundamental nature of optical beams that do not interact with each other even in the case of crossing of their light path. One of the early inventions has been disclosed in U.S. Pat. No. 3,623,026 (1971), where a semiconductor device utilized for optical storing and reading of information has been proposed. This method utilizes a conductor-insulator-semiconductor structure (CIS) that serves as a capacitor for data storage. A thin layer of insulating material separates the conductor from the semiconductor. When the CIS structure is charged to the predetermined voltage and exposed to the radiation of band gap energies passing through the substantially transparent conductor and insulator layers, minority carriers are generated in the semiconductor bulk near or in the depletion region and move to the insulator-semiconductor interface. Reversing the voltage changes the direction of the electric field, thereby injecting minority carriers into the semiconductor and causing an emission of electromagnetic radiation. New approaches for an efficient and low cost technology are disclosed in U.S. Pat. No. 5,504,323, in which the device combines functions of the light emitting diode and photo-receiver. In this case, a positively biased diode functions as a light emitter. When a negative bias is applied, it becomes a highly efficient photo-diode. The described devices are an example of volatile memory, which needs continuous power supplying. A non-volatile optical semiconductor memory device was proposed recently U.S. Pat. No. 6,147,901 (2000). In this case, a memory cell utilizes vertically stacked structures comprised of p-i-n-i-p or n-i-p-i-n structures, where p means p-type semiconductor, n is n-type semiconductor and i is the intrinsic type of semiconductor. The electron-hole pairs are generated under the light illumination in the p/n-junction zone and under the biased voltage, electrons tunnel in the i-type semiconductor and are trapped there by the impurities. Such a structure is known as an electrical write-erase non-volatile memory.
The references mentioned above disclose types of memory cells and do not suggest any write/read device. PCT International Application No. WO 97/48009 A1 (1997) suggests an optical logic element that comprises a light source, a memory sub-layer, and a photo-sensor sub-layer. A plurality of these elements are assembled in memory or logic layers that could be integrated into a 3D multi-layer device. As a material for the memory device, the reference suggests a wide variety of materials which change their optical properties when exposed to the illuminating light source. These materials could be liquid crystals, photo-chromes or photo-chemicals. However, a possible realization of this idea is rather problematic due to the limited sensitivity of the photosensitive sub-layer. For example, the most advanced modem CCD matrix has a total charge capacity per pixel of about 105 electrons in a size of 10 xcexc. In the pixel of the proposed device having the size of 0.5 xcexc, this value will be reduced to 250 electrons, which means that the working average value of the charge is about 100 electrons. During the reading, this amount of electrons corresponds to the shot noise of 10%, which is unacceptable for most of the applications.
The above described technical barriers exist today in the miniature high capacity semiconductor memory devices that are capable of storing and reading information with a high data rate due to the processes of the absorption and emission of light
The present invention resolves the above described limitations and has a way to provide a new type of solid state semiconductor memory devices which can retain stored content for a certain time period even after removal of the power supply. This is achieved by employing methods of light absorption and emission.
Another object of the present invention is to provide a semiconductor memory device capable of writing data optically and erasing data electrically and/or optically with a high rate of speed at the same time preserving the stored content even after removal of the power supply.
A further object of the present invention is to provide a semiconductor memory device capable of reading data optically at a high rate of speed.
A further object of the present invention is to provide a semiconductor memory device capable of being manufactured with the relatively easy fabrication processes and at lower costs.
A further object of the present invention is to provide a semiconductor memory device having a simple structure capable of being miniaturized with a relative ease.
A further object of the present invention is to provide a combination of the different types of semiconductor memory devices such as DRAM, SDRAM, and PROM combined with the functions of a typical CPU such as arithmetic and logic units. All of the functions in such types of devices could be combined in a single chip.
To achieve the required objectives, a first feature of the present invention lies in the arrangement of a sub-layer comprised of a two-dimensional array of electroluminescent cells organized into rows and columns, which are individually electrically addressable through the system of transparent electrodes disposed in rows and columns. The electroluminescent cells are located between the crossings of the electrodes. The electroluminescent materials could be organic or non-organic semiconductors. Each electroluminescent cell is stacked with a vertical multi-layer structure of memory cell matrices in which biased voltage can be applied through the crossing system of optically transparent electrodes to all memory cells in each layer. It should be noted that the memory cells are not individually electrically addressable; thus, each memory cell layer does not need such electronic circuits like parallel shift registers and thus it has a very simple structure. Each memory cell uses a CIS structure, which is a modification of the memory cell used in the electrically write/erase non-volatile semiconductor memory devices such as flash memory. Usually, the CIS structure is utilized as a capacitor to store information in the form of a charge. Such devices can be separated into a broad range defined by the charge storage mechanism. The first class contains devices where charges are stored in the deep energy states at or near the interface between the two gate dielectrics. In the devices of this class, the conducting mechanism utilized to transfer charges to the storage sites from the substrate is known as xe2x80x9ctunneling.xe2x80x9d Tunneling is also utilized to remove charges from the storage sites to their original positions by the application of a control voltage pulse of the polarity opposite to that utilized in transferring and storage of the initial charges. The tunneling flux depends on the concentration of the minority carriers. The illumination of the semiconductor bulk by light having a frequency greater than the semiconductor band gap generates electron hole pairs in the controllable way. The number of the minority carriers generated is proportional to the amount of integrated radiation flux. When the reversed biased voltage is applied, trapped minority carriers tunnel back to the bulk semiconductor where they recombine with the minority carriers of the opposite type; that recombination is accompanied by a light emission. The other class of these memory devices could contain structures in which the charge is trapped in floating electrodes buried into dielectric. The best known device of this class is the FAMOS type (floating gate-avalanche injection MOS). In a FAMOS device, charges are transferred from a substrate to the floating gate by the avalanche breakdown that occurs under the high voltage applied between the source and drain. High-energy electrons are generated in the depletion regions of the reversed biased p/n junctions and pass through the gate dielectric material reaching floating gate under the influence of a strong electric field of the p/n junction. The floating gate could be a silicon-nitride thin layer that prevents conducting of electrons, yet it allows for a xe2x80x9cholexe2x80x9d conductance. The conducting gate-electrodes are isolated by the insulator layers, which are made of silicon nitride. The xe2x80x9chotxe2x80x9d electrons are trapped in the silicon nitride layer. In the modified electro-optical variant of the FAMOS device, the avalanche breakdown is initiated by the illuminating light, which generates the xe2x80x9celectron-holexe2x80x9d pairs. The accumulation of the minority carriers in the p/n junction zone under the reverse biased voltage leads to a breakdown of the junction and trapping of the charges in the buried gate. To read the information written in the memory cell a relatively large positive voltage is applied to the gate contacts. The xe2x80x9cholesxe2x80x9d from the gate contact are injected into the nitride and attract negatively trapped charges. Recombination of the holes and trapped electrons results in the light emission. After this, the restoring voltage pulse recharges the cell. Other materials exhibiting such characteristics may also be suitable as well. It could be for example an organic polymer semiconductor. As it is mentioned above, the memory cell could exploit the organic polymer heterostructure, which serves as a photo-receiver and light emitter simultaneously. In another variant, the memory layers could be of constructed of electrochromic materials which change their transparency under the biased voltage. In the case of the nonvolatile memory, the transparency pattern exists when the biased voltage is turned off. Erasing of information from the layers is obtained by the application of the higher biased voltage. In the case of a WORM device, the changes in the transparency are made irreversibly. In the ROM device, the information is recorded in the form of pits filled by electrochromic material. The electrochromic medium works in the following way. When the biased voltage is turned off, all of the memory layers are totally transparent. The spatial modulation of the transparency occurs when the biased voltage is turned on. The system of crossing electrodes forms the pattern of transparency, which serves as a mask for the illuminating light. The broad variety of the design of the chargeable cells could be applied as well. For example, the memory cell could consist of two parts one of which is chargeable and the other is electroluminescent. During the reading process, the electroluminescent sub-cell may emit light. This depends on whether or not the chargeable sub-cell is charged up. Note that the sub-cells could be located in the same plane or they could be vertically stacked. In another embodiment, the optically transparent electroluminescent matrix could be located in the middle of the multi-layer stack. In this case, the electroluminescent matrix would emit light to the upper and lower semi-spheres simultaneously. The process of reading could be realized by the photosensitive matrix at the lower or upper boundary of the stack or by two photosensitive matrixes located at the upper and lower boundaries of the stack. A particular advantage of the optical writing and reading layers in the compatibility with vertical configuration of memory cell, when the charge coupling occurs between vertically disposed potential wells. This design reduces the size of the individual cell and leads to a simpler type of circuits. The smaller the size of the circuit, the more economical it is to manufacture a memory device.
The plurality of the compounds consisting of the electroluminescent matrix and stack of chargeable layers are isolated by the transparent insulating layers and stacked with the photo-sensitive matrix which could be CCD or CMOS device. The individual pixel of the photosensitive matrix has a size typical for a standard device of about 5-10 xcexc. One advantage of various embodiments of the present invention is a lensless collection of the emitted light. Because of it, the cross talk between the emitting cells could be very significant. However, the demands for the contrast of the information image originated from the SNR are rather liberal, as it will be shown below that the contrast of about 8% can be sufficient. In order to avoid the undesirable cross talk between the simultaneously activated emitting memory cells, the distance between them must be more than the size of the photosensitive pixel. Evidently, the cross talk between the neighbor emitting memory cells depends on the distance between the photo-sensor surface and a certain sub-layer of the memory cell. The larger the distancexe2x80x94the more cross talk. Thus, the thickness of the memory stack can not be too large. This is one of the main factors limiting the capacity of this memory device. The optimal total thickness of the multi-layer memory structure must be comparable with the distance between the activated cells. This means that in the different memory layers the distance between the simultaneously activated emitting cells is different. The readout process could be realized from different layers simultaneously. The cross talk is avoided by the proper choice of the readout zone. This way the data reading could have a content protection feature which consists of a code recorded in a specific layer which gates data outflow from the device. To avoid the undesirable cross talk between the neighbor electroluminescent cells during recording, the distance between the cells as well as the distance between the activated memory cells should be sufficiently large. It should be noted as well that in this case the electroluminescent cells could be significantly larger in size than the memory cells